Klystrons are a class of amplifier in which a signal to be amplified is coupled into an input resonant cavity and produces an electric field which acts on electrons of the beam to modify their velocity. Some electrons are accelerated whilst others are retarded, producing bunching (that is, density modulation of the beam) at some position further down the tube. Typically, there are several subsequent resonant cavities to increase the amount of bunching and a final resonant cavity at which the amplified signal is extracted.
Another type of amplifying arrangement, known as a klystron-tetrode, also utilises modulation of an electron beam to obtain amplification of an applied signal. In this case, a density modulated beam is produced by applying an input signal to be amplified to a modulation grid positioned near to and in front of an electron gun cathode. Emission of electrons is alternately inhibited and allowed, producing bunching of electrons. The bunched beam is accelerated down the tube to an output resonant cavity at which an amplified signal is extracted.
For both klystrons and klystron-tetrodes the input signal which produces bunching of electron beam is a radio frequency (RF) signal, that is, a signal oscillating in the range 470 to 860 MHz. For television requirements, the input carrier signal is modulated in amplitude and by having a relatively low frequency colour sub-carrier imposed on it, these modulating signals being known as video signals. The colour sub-carrier has a frequency of the order of 4 MHz, the exact value depending on the type of television standard being used.